1. Field of the Invention
The present invention relates to a vacuum casting method of the type wherein a molding cavity is reduced in pressure to a vacuum and upon opening a gate, a molten metal is charged into the molding cavity at a high speed. More particularly, the present invention relates to an improved vacuum casting method in which bubbles and solid metal pieces are prevented from being involved in the molten metal as it is charged into the molding cavity.
2. Description of Related Art
As one example of a light alloy casting method of a high quality and a low cost, a vacuum precharged closed squeezed casting method was proposed by the present applicant in Japanese Patent Application No. HEI 4-309534 filed on Oct. 23, 1992.
In the proposed vacuum casting method, a molding cavity is shut off from an interior of a molten metal retaining dome by a gate. Then, the molding cavity is reduced in pressure to a vacuum, and substantially simultaneously a portion of a molten metal held in a molten metal holding furnace is raised to the molten metal retaining dome. Thereafter, the gate is opened so that the molten metal in the molten metal retaining dome is charged into the molding cavity at a high speed due to the vacuum in the molding cavity. The molding cavity is then shut-off by a shut pin, and the molten metal in the molding cavity is pressurized by inserting a pressure pin into the molding cavity. Thereafter, the molten metal in the molding cavity is cooled to be solidified.
In the proposed vacuum casting method, since the molding cavity is reduced in pressure to a vacuum before the molten metal is charged into it, the molten metal has few or no bubbles once in the molding cavity. But, because of the vacuum generated in the molding cavity, the charging speed of the molten metal is very high. The vacuum casting method allows the molten metal to run smoothly in the molding cavity and, as a result, slimmer and lighter casting products are possible.
However, to realize a higher quality of casting using the above-described vacuum casting method, it is necessary to prevent the molten metal from involving air when it rises in the molten metal retaining dome via a stalk from a molten metal holding furnace, because the molten metal contacts air in the stalk and in the molten metal retaining dome.